- fixes#1079 where partitioning is happening twice in the case of
`strategy="ocr_only"`
- only calls `extractable_elements` if we can predetermine that
`ocr_only` is not a possible strategy even if it was the intended
strategy.
- Adds additional assertion test that `_partition_pdf_or_image_with_ocr`
is not called when falling back to `fast` from `ocr_only`